With the rapid progress of the multimedia society, the technologies of semiconductor elements and display apparatus have dramatic progress accordingly. In terms of the display apparatus, an active matrix organic light emitting diode (AMOLED) display device meets specification requirements for a display in multimedia age due to its advantages of free viewing angle limitation, low manufacturing cost, high response speed (approximately more than a hundred times of a liquid crystal display), power saving, self-luminescent, direct current driver that can be used for a portable machine, wide operating temperature range, light weight, and miniaturization and thinning according to a hardware device. Therefore, the AMOLED display device has great potential for development, and is expected to become a next generation of a new-model flat panel display so as to take the place of a liquid crystal display (LCD).
At present, an AMOLED display panel has mainly three modes of manufacturing. The first mode is to manufacture by utilizing amorphous silicon (a-Si) thin film transistor (TFT) process technology. The second mode is to manufacture by utilizing low temperature poly-silicon (LTPS) TFT process technology. The third mode is to manufacture by utilizing oxide TFT process technology. Herein, compared to other two modes, a-Si TFT does not have a relatively high electronic mobility, and its threshold voltage would produce shift under a long time of compression and high temperature, which would result in non-uniformity of the display panel. Generally, the a-Si TFT is applied on the LCD widely. However, the LTPS TFT process technology needs to adopt multi-channel mask manufacturing process, thereby resulting in an increase of cost. Therefore, the LTPS TFT process technology is mainly applicable to small and medium size panels currently, while the oxide TFT process technology is mainly applicable to a large-size AMOLED panel.
Generally, for an AMOLED display panel manufactured by adopting the oxide TFT process technology, types of TFTs in its pixel circuit can be divided into P-type or N-type. However, no matter a P-type TFT or an N-type TFT is selected to realize an organic light emitting diode (OLED) pixel circuit, a turn-on voltage (Voled_th) of the OLED would change under the effect of long time stress, while current flowing through the OLED would not only vary with the turn-on voltage (Voled_th) of the OLED, but also be different with threshold voltage shift (Vth shift) of TFT used for driving the OLED. As such, brightness uniformity and brightness constancy of the OLED display apparatus would be also influenced.